Core dumps are disabled by default under Darwin/Mac OS X. To re-enable
core dumps, a privileged user must do one of the following
* Edit /etc/launchd.conf or $HOME/.launchd.conf and add a line specifying
the limit limit core unlimited
* A privileged user can also enable cores with launchctl limit core
unlimited
* A privileged user can also enable core files by using ulimit(1) or
limit(1) depending upon the shell.

In other words, you might want to try `sudo launchctl limit core unlimited` followed by a logout/login. I haven't tested that, so there might be more to it... Also, you could just run 4ed from lldb, it can manually generate a core file as well as dump a stack trace...